at least for Germany: digital invoices shall be processed in a way that manipulation can be ruled out (GoBD). Hence companies use special scanner and document management systems which document the Revisionssicherheit.
And that is the new, more relaxed situation. At first, digital invoices were required to be signed with a qualified signature, i.e. spend a few hundred quid a year on certificates by a few select CAs (only the usual german "suspects" could qualify due to intentionally onerous requirements).